1. Field of the Invention
The present invention relates to a method of encapsulating an electronic part such as an IC formed on a substrate with a thermosetting resin.
2. Description of the Prior Art
Recently, an IC card having an IC chip such as a CPU or a memory chip has been rapidly developed. Since such an IC card has a larger memory capacity than that of a conventional magnetic card, it is proposed to use the IC card in place of a bankbook or a credit card.
A conventional IC card and an IC module housed therein will be described below with reference to FIGS. 1 to 7.
As shown in FIG. 1, a plurality of data input/output terminals 33a are formed on the surface of an IC card 30. The overall shape of the IC card 30 is defined by a plastic card base 31 shown in FIG. 3.
As shown in FIG. 2, a recess portion 31a is formed in the card base 31. An IC module 32 in which an IC chip 33b such as a CPU or a memory chip is bonded on a substrate 33 and a resin encapsulating portion 34 is formed to cover the IC chip 33b is housed and fixed in the recess portion 31a.
After the IC module 32 is housed and fixed in the recess portion 31a of the card base 31, over sheets 35 are covered on both the surfaces of the card base 31, thereby constituting the IC card 30.
Referring to FIG. 2, the upper surface of the encapsulating portion 34 abuts against the bottom surface of the recess portion 31a to define the thickness of the IC module 32, and the outer circumference of the substrate 33 engages with the inner circumference of the recess portion 31a to define its horizontal position.
The IC card 30 generally has a thickness of about 0.8 mm and requires flexibility against bending to some extent so as not to be broken by an external force applied when it is carried or used. For this reason, the card base 31 must have flexibility, and the IC module 32 must be encapsulated with a resin having high stiffness and humidity resistance so that the IC chip 33b is not broken or a bonding wire is not disconnected by bending.
In addition, since the upper surface of the encapsulating portion 34 abuts against the bottom surface of the recess portion 31a of the thin card base 31 to define the thickness, the size of the encapsulating portion 34 must be controlled to prevent thickness variations. In a conventional potting encapsulating method, however, which has been widely used and in which a molten resin is dropped on the IC chip, the thickness of the encapsulating portion 34 tends to vary. Therefore, a transfer molding method using a thermosetting resin having high dimensional precision and stiffness is best suited to formation of the encapsulating portion 34 of the IC module.
A method of resin-encapsulating an IC chip according to conventional transfer molding will be described below with reference to FIG. 4.
Referring to FIG. 4, reference numeral 36 denotes a lower half mold. The substrate 33 on which the IC chip 33b is bonded is placed on the upper surface of the lower half mold 36. Reference numeral 37 denotes an upper half mold in which a cavity 37a is formed. The upper half mold 37 is fixed on the lower half mold 36 so that the cavity 37a covers the upper surface of the IC chip 33b.
A gate groove 39a and a runner groove 40a are formed in the upper half mold 37. A runner 40 and a gate 39 for guiding a resin 38 to the cavity 37a are formed between the faces of the upper and lower half molds 37 and 36 and between the faces of the upper half mold 37 and the substrate 33, respectively. This arrangement corresponds to a so-called side gate type transfer molding method.
In order to form the encapsulating portion 34, the heated and melted resin 38 is injected into the runner 40 by a plunger 41 and filled in the cavity 37a through the gate 39.
When the resin 38 is set, the upper half mold 37 is removed to extract the IC module 32 in which the encapsulating portion 34 is formed. In this case, however, the gate 39 is formed between the faces of the upper half mold 37 and the substrate 33 as described above. Therefore, as shown in FIG. 5, a gate remaining portion 34a connected to the encapsulating portion 34 is formed on the substrate 33 of the IC module 32.
The gate remaining portion 34a has high adhesion strength with respect to a surface 33c of the substrate 33. Therefore, when a gate remaining portion 34a' indicated by an alternate long and dashed line is to be bent and removed, a bending force acts on the gate remaining portion 34a to remove the surface 33c of the substrate 33. In the worst case, even a circuit pattern is disconnected.
In order not to remove the gate remaining portion 34a', a groove 31b for receiving the gate remaining portion 34a must be additionally formed in the recess portion 31a of the card base 31.
Since the card base 31 is made of a plastic material as described above, the recess portion 31a and the groove 31b are molded at the same time the card base 31 is formed by injection molding. When the recess portion 31a and the groove 31b are to be simultaneously formed in the card base 31 by injection molding, the plastic material in an injection mold flows not through the groove 31b but through portions indicated by arrows B and C as shown in FIG. 3. As a result, the respective plastic flows are incompletely bonded to each other at a position indicated by a broken line D.
As described above, the IC card 30 requires the flexibility against bending If the above plastic incomplete bonded portion D is formed in the card base 31, however, a crack is produced from the incomplete bonded portion D even by slight bending to break the card base 31.
In order not to form the groove 31b in the card base 31, therefore, a demand has arisen for a transfer molding method which can encapsulate the IC chip 32 without forming the gate remaining portion 34a on the substrate 33 of the IC module 32.
Japanese Examined Patent Publication (Kokoku) No. 61-46049 discloses a resin encapsulating method which can eliminate the drawback of the above conventional resin encapsulating method.
This resin encapsulating method uses a cavity plate shown in FIG. 6 In addition to positioning pilot holes 42a, a cavity 42b for defining the size of an encapsulating portion is punched in a cavity plate 42. In this resin encapsulating method, as shown in FIG. 7, the substrate 33 on which an electronic part to be encapsulated with a resin is mounted and the cavity plate 42 in which the cavity 42b for defining the position and size of a resin encapsulating portion are interposed between the upper half mold 37 and the lower half mold 36. An encapsulating resin 43 is filled in the cavity through the runner 40 and the gate 39 formed in the upper surface of the lower half mold 36 to be brought into contact with the cavity plate 42. That is, this method is a so-called side gate type transfer molding method.
Note that the gate 39 is located at the upper opening end of the cavity 42b.
According to the above resin encapsulating method, the gate 39 for guiding the encapsulating resin is formed not on the substrate 33 but on the encapsulating portion 34. Therefore, since the gate remaining portion 34a is not formed on the substrate 33 as indicated by the alternate long and dashed line in FIG. 8, the conventional problem shown in FIG. 5 can be solved.
In the above resin encapsulating method, however, the gate groove is positioned at the upper opening portion of the cavity, i.e., the upper end face of the encapsulating portion 34 when the encapsulating resin is filled in the cavity. Therefore, as shown in FIG. 8, the gate remaining portion 34b projects from the upper end face of the encapsulating portion 34.
As shown in FIG. 9, if the IC module 32 having the gate remaining portion 34b is housed in a recess portion 43a. of a card base 43, the gate remaining portion 34b abuts against the bottom surface of the recess portion 43a to form a gap G between the upper surface of the encapsulating portion 34 and the bottom surface of the recess portion 43a. As a result, the thickness of the card is undesirably increased larger than a predetermined thickness at a portion of the IC module 32.
For this reason, the gate remaining portion 34b must be removed by a file after the encapsulating portion 34 is formed. This troublesome operation poses a problem of reliability.
The above resin encapsulating method also has a problem of maintenance of a mold. That is, in order to improve stiffness of the encapsulating portion 34, glass fibers are often mixed in the encapsulating resin.
In this case, since a thin gate groove is strongly worn by the glass fibers, a member having the gate groove must be periodically replaced. According to the above resin encapsulating method, however, since the gate groove is formed in the lower half mold, the expensive lower half mold must be entirely replaced when the gate groove is worn, resulting in a high mold cost.
As countermeasures against wear of the gate groove, a variety of resin molding methods in which only a part of a mold having a gate groove which is strongly worn can be replaced are proposed as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-13312.
In this resin encapsulating method, a sprue having a hole for injecting a resin is constituted by a replaceable bushing, and this sprue bushing is located at a stationary insert die. When a cavity formed between the faces of stationary and movable insert dies communicates with the hole of the sprue bushing, a molten resin is injected from an injection cylinder to the hole of the sprue bushing, thereby filling the resin into the cavity. That is, this method is a so-called pin gate type resin molding method.
After the resin is filled and set in the cavity, a molded product and the resin set in the sprue bushing are removed, and resin molding is performed again.
In this resin molding method, since the sprue as a resin injection hole which is worn most strongly is constituted by the replaceable bushing, only the sprue bushing can be easily replaced when the sprue is strongly worn.
In this resin molding method, however, the following difference occurs between molding using a thermoplastic resin and that using a thermosetting resin.
That is, since the thermoplastic resin has a high thermal shrinkage rate, it can be easily released from a mold. In addition, the thermoplastic resin can maintain its toughness even after it is set. Therefore, even a resin set in the hole of the sprue bushing can be easily extracted and removed therefrom.
In contrast, since the thermosetting resin has a low thermal shrinkage rate, it adheres on the surface of a mold and is not easily released therefrom when it is set. In addition, the thermosetting resin almost loses its toughness after it is set. Therefore, when a set resin is to be extracted from the hole of the sprue bushing, the set resin is broken in the hole of the sprue bushing and the resin remains therein.
For this reason, since it is difficult to use the thermosetting resin in this pin gate type resin molding method, the above side gate type transfer molding method is conventionally used.
The resin encapsulating method disclosed in Japanese Examined Patent Publication (Kokoku) No. 61-46049 further has the following problem.
That is, as described above, the thermosetting resin has a low thermal shrinkage rate and therefore is not easily released from a mold. Therefore, when the encapsulating resin is filled in the cavity of the cavity plate for defining the shape of the encapsulating portion to encapsulate the electronic part on the substrate, the encapsulating portion and the cavity plate are kept adhered with each other after the encapsulating resin is set. Therefore, in order to remove the substrate having the encapsulating portion thereon, the substrate with the cavity plate adhered thereto is removed from a mold, the substrate and the cavity plate are manually separated from each other, and then the cavity plate is mounted in the mold. This series of operations must be manually performed because it is difficult to automatically perform them, resulting in very poor production efficiency.